1. Field of the Invention
The present invention relates to a method for producing a polymer structure on a substrate surface.
2. Description of Related Art
In semiconductor devices, circuit structures are arranged in cavities to an increasing extent. The cavity serves to provide a clearance above a circuit structure formed in a substrate. This clearance is necessary for a correct functionality of the circuit structure implemented in the substrate. These circuit elements may, for example, comprise microelectromechanical systems, piezo-electric resonators, such as, for example, BAW or bulk acoustic wave filters.
Such a cavity may, for example, be formed in a polymer structure. In order to produce such a cavity, large structures having a high aspect ratio, i.e. a high ratio of a height or thickness of the polymer structure to a length or width of the polymer structure on a substrate are often required. Here, aspect ratio means the ratio of the height to the length or width of the cuboid-shaped polymer structure. An SU-8 photoresist or photo epoxide, for example, is used here as a polymer. Adhesion problems arise on many foundations or surfaces of the substrate. These adhesion problems result from the stress forming when curing or heating the polymer structure through the accompanying shrinking of the polymer structure. Curing the polymer structure here is preferably performed after patterning the polymer structure. The adhesion problems cause polymer layer or polymer structure peeling, wherein the peeling of the polymer structures frequently takes place at the edges and, in particular, at the corners of the polymer structure.
This may result in the polymer structures peeling off the surface of the substrate over the lifetime of such an assembly, such as, for example, a semiconductor device having a circuit structure in a cavity. A potential consequence is for the electrical device implemented in this way to no longer have the correct electrical functionality and thus to be destroyed. Consequently, the adhesion problems of the polymer structure on the substrate entail potential reliability problems of the electrical devices implemented in this way.
The adhesion problems discussed above may, even in manufacturing, result in the polymer structures peeling off the substrate surface, wherein the manufacturing yield is decreased in mass production where polymer structures are deposited onto a substrate surface. This entails an increase in manufacturing costs for the products employing a polymer structure deposited onto the substrate surface.
A way of increasing adhesion between the polymer structure and the substrate surface is the usage of so-called primers. They are frequently based on silanes or organosilanes belonging to the group of substances of organosilicon compounds consisting of silicon atoms, a hydrolysable group and an organofunctional group. Organosilanes are generally employed as primers between an inorganic surface, such as, for example, the surface of a substrate, and a polymer or plastic. These primers, such as, for example, aminosilane, react with hydroxyl groups (—OH) on the substrate surface and thus form, with the end facing away from the oxygen molecule, a coupling molecule to the polymer structure or a polymer molecule in the polymer structure.
A disadvantage of using the primer is that the increase in adhesion depends on a presence of oxygen on the surface of the polymer. This makes the manufacturing method for substrates having polymer structures on the surfaces more complicated. With polymer structures or materials where there is little or no oxygen on the surface, this method does not result in a sufficient increase in the adhesion of the polymer structure.
A further disadvantage of using primers for increasing adhesion of a polymer structure on a substrate surface is that primers are of high stability. At the same time, primers on substrate surfaces have negative effects in a series of electrical devices, such as, for example, bulk acoustic wave filters. In bulk acoustic wave filters, the primers on the substrate surface may, for example, result in an undesired shift of the frequency response, requiring the primer to be removed selectively.
In many cases, removing the primer can only take place using strong etching agents or solvents, such as, for example, strongly oxidizing acid mixtures or lyes. Here, removing the primer may result in etching or removing material at a sensitive position of the substrate surface, wherein the functionality of a circuit structure implemented in the substrate may, for example, be impeded.
Consequently, removing material on active sensitive surfaces of the substrate, such as, for example, on a surface of a bulk acoustic wave filter, may result in influencing the electrical behavior of the bulk acoustic wave filter and may thus, for example, change the frequency response of the bulk acoustic wave filter. This may result in even the resonant frequency of the bulk acoustic wave filters implemented in the substrate to change, wherein the resonant frequency might even no longer be in the specified range. The bulk acoustic wave filters, the resonant frequency of which is outside the specified range, are not suitable for being employed in an electrical device and are consequently frequently disposed of after testing the bulk acoustic wave filters. The result is that the manufacturing yield is reduced, which at the same time entails an increase in the manufacturing costs.